1. Field of the Invention
This invention relates to processes for hydrolyzing carbohydrate polymers such as starch and cellulose and more particularly, to such processes especially adapted to provide substrate sugars for the fermentation of ethanol.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbobydrate plant matter and other extraneous substances, the acid and/or enzymatic hydrolysis of starch and/or cellulose to fermentable sugar (saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum-based chemicals. To date, however, relatively little concern has been given to the energy requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure for carrying out any of the discrete operations involved in the manufacture of ethanol from vegetative sources.
The substitution of alcohol for at least a portion of petroleum-based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources. The most common such operation employs cane sugar in a fermention-distillation operation which conveniently utilizes the bagasse by-product as a fuel source. Cassava or manioc (Manihot utilissima Pohl) as a source of starch has also been considered for conversion into alcohol (see "Brazil's National Alcohol Programme", Jackson, ed (Process Biochemistry pp. 1781-1783 (1950); and United Kingdom Pat. No. 1,277,002). However, since manioc lacks the equivalent of sugar cane's bagasse, the fuel for alcohol conversion must come from an external source. Thus, to make manioc root starch or for that matter, any carbohydrate polymer, an economically attractive source of ethanol, it is essential to achieve rapid and high levels of conversion of the carbohydrate polymer to fermentable saccharide and of the fermentable saccharide to ethanol with high levels of thermal efficiency and at low plant investment and operating costs.
Numerous processes for the acid hydrolysis of cellulose and starch to provide fermentable saccharides are known (viz., the acid starch hydrolysis processes described in U.S. Pat. Nos. 2,203,325; 2,210,659; 3,359,763; 2,393,095; 2,395,907; 2,565,404; 2,946,706; 2,954,304; 2,989,425; 3,169,083; 3,200,012; 3,236,687; 3,313,654; 3,446,664; 3,484,287; 3,607,395 and 4,137,654 and, the acid cellulose hydrolysis processes described in U.S. Pat. Nos. 1,096,030; 1,242,030; 1,355,415; 1,428,217; 1,641,771; 1,697,785; 1,806,531; 1,828,982; 1,890,304; 1,919,623; 1,936,190; 1,936,972; 1,969,646; 1,990,097; 2,008,284; 2,086,701; 2,086,963; 2,088,977; 2,108,567; 2,220,846; 2,239,095; 2,284,500; 2,302,022; 2,305,833; 2,356,500; 2,426,677; 2,450,586; 2,474,669; 2,488,981; 2,516,833; 2,681,871; 2,735,792; 2,739,086; 2,752,270; 2,759,856; 2,778,751; 2,801,939; 2,835,611; 2,851,382; 2,900,284; 2,945,777; 2,951,775; 2,959,500; 2,974,067; 3,067,065; 3,212,932; 3,212,933; 3,251,716; 3,479,248; 3,523,911; 3,640,768; 3,787,241; 4,018,620; and 4,029,515).
The acid hydrolysis of manioc root starch with mineral acid preparative to fermentation of the resulting sugar to produce ethanol has been investigated (see "Tapioca as a Source of Alcohol", Krishnamurti, E. G. 1960, Current Science 9:346-348). However, the hydrolysis could not be effected without the addition of fresh acid. Moreover, under the conditions employed (heating at 50-60 p.s.i. with 2% sulfuric acid for 4 hours), the hydrolyzed starch solution developed a dark color and a burnt smell which would signal saccharide degradation. Hydrolysis of the starch with lower amounts of sulfuric acids, i.e., 0.5% and 1.0% respectively, under the foregoing conditions failed to provide complete hydrolysis.
Processes for the continuous fermentation of sugars to provide alcohol are well known (viz., U.S. Pat. Nos. 2,155,134; 2,371,208; 2,967,107; 3,015,612; 3,078,166; 3,093,548; 3,177,005; 3,201,328; 3,207,605; 3,207,606; 3,219,319; 3,234,026; 3,413,124; 3,528,887; 3,575,813; 3,591,454; 3,705,841; 3,737,323; and 3,940,492; "Process Design and Economic Studies of Alternative Fermentation Methods for the Production of Ethanol", Cysewski, et al. Biotechnology and Bioengineering, Vol. xx, pp. 1421-1444 (1978)). In a typical continuous fermentation process, a stream of sterile sugar liquor and a quantity of yeast cells are introduced into the first of a battery of fermentation vessels wherein initial fermentation takes place, generally under conditions favoring rapid cell growth. The partial fermentate admixed with yeast cells is continuously withdrawn from the first fermentation vessel wherein fermentation is carried out under conditions favoring the rapid conversion of sugar to ethanol. The yeast in the last fermentation vessel can be recovered by suitable means, e.g., centrifugation or settlement, and recycled. In such a system, the ability of the fermentation organism to produce ethanol is affected by the ethanol and sugar concentrations. As a rule, a yeast which gives high conversion rates of sugar to ethanol in a low-ethanol, high-sugar fermentation medium will only sluggishly produce ethanol under the opposite conditions, i.e., at high-ethanol level, low-sugar concentrations.
Accordingly, there has heretofore existed a need for a process for the acid hydrolysis of carbohydrate polymers from cellulose and/or starch at rapid and high levels of conversion without any significant degradation of the resulting saccharide and at only a modest expenditure of thermal energy and of utilizing the saccharide in a thermally efficient, rapid continuous fermentation process to provide industrial ethanol at competitive prices.